The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Conventional electrostatic spinning systems consist of a hollow needle spinneret, a pump for feeding the polymer solution through to the spinneret, an electrode collector plate and a high voltage power supply connected between the spinneret and the collector plate. The polymer liquid is fed to the spinneret, and charged with a high voltage. The resulting electrostatic force between the spinneret and the collector plate draws the solution into a Taylor cone. If the liquid has sufficient cohesion, the liquid is drawn out as a jet. Interaction between the jet, the external electric field and charge repulsion inside the jet causes the jet to bend and spin and therefore stretch thinner. Solvent evaporation causes the jet to solidify and be randomly deposited on the collector plate to form a non-woven nanofibre mat.
However, this production system provides limited fibre productivity as each spinneret only generates a single polymer jet producing up to 300 milligram of fibre per hour per needle. The small needle diameter also results in a highly concentrated electric field near the spinneret surface which is susceptible to corona discharge under a high applied voltage. The voltage is therefore generally restricted to below 30 kV. The low operating voltage also leads to coarse nanofibres.
The rate of production of nanofibres can be increased by electrostatically generating nanofibres from a broad liquid surface. For example, International patent publication WO2005024101 discloses an electrospinning device including a spinning cylindrical electrode (or “spinneret”) that is partially immersed in a reservoir of polymer solution. A counter electrode is located a distance away from the cylindrical electrode. The polymer solution is carried as a thin film from the reservoir on the surface of the cylinder into the electric field between the spinning and collecting electrode. Nanofibres are created at certain points on the surface when the electrostatic field intensity between the electrodes is sufficient to draw the solution into a Taylor cone. The resultant nanofibres are randomly deposited on the counter electrode to form a non-woven nanofibre mat.
The formation of jets and resultant fibre morphology in these types of large scale electrospinning arrangements is highly influenced by the electric field intensity around the spinneret and the electric field intensity profile in the electrospinning zone. For example, the length of cylindrically shaped electrodes influences the quantity and critical voltage required to produce jets across the entire surface of the cylinder. At some voltages, jets are only produced at the ends of the cylinder. The thickness of the nanofibres produced over the length of the cylinder can also vary due to variations in the electric field intensity along that length.
It would therefore be desirable to provide an alternate spinneret for an electrostatic spinning device for production of nanofibres that address one or more of the above disadvantages.